Polymeric piezoelectric material, and process for producing the same

ABSTRACT

The invention provides a polymeric piezoelectric material including a helical chiral polymer having a weight-average molecular weight of from 50,000 to 1,000,000 and having optical activity, wherein a crystallinity of the material measured by a DSC method is from 20% to 80%, and a product of a standardized molecular orientation MORc measured by a microwave transmission type molecular orientation meter based on a reference thickness of 50 μm and the crystallinity is from 25 to 250.

TECHNICAL FIELD

The present invention relates to a polymeric piezoelectric material anda process for producing the same.

BACKGROUND ART

As a piezoelectric material a ceramic material of PZT (PBZrO₃-PbTiO₃type solid solution) has been heretofore broadly used. However, sincePZT contains lead, a polymeric piezoelectric material, which imposesless environmental burden and has higher flexibility, has been currentlycoming into use as a piezoelectric material.

Currently known polymeric piezoelectric materials can be classifiedroughly into 2 types. Namely, into 2 types of poled polymers, asrepresented by nylon 11, polyvinyl fluoride, polyvinyl chloride,polyurea, etc. and ferroelectric polymers, as represented by (β-type)polyvinylidene fluoride (PVDF), a vinylidene fluoride-trifluoroethylenecopolymer (P(VDF-TrFE)) (75/25), etc.

However, polymeric piezoelectric materials are inferior to PZT in termsof piezoelectricity, and therefore improvement of the piezoelectricityhas been demanded. In order to cope with the above, attempts atimprovement of the piezoelectricity of polymeric piezoelectric materialshave been made from various viewpoints.

For example, PVDF and P(VDF-TrFE), which are ferroelectric polymers,have superior piezoelectricity among polymers and a piezoelectricconstant d₃₁ of 20 pC/N or higher. A film material formed from PVDF orP(VDF-TrFE) is imparted with piezoelectricity by orientating polymerchains by a stretching operation in the stretching direction; thenbuilding up opposite electric charges on the back and front sides of thefilm by means of corona discharge, etc. to generate an electric fieldperpendicular to the film surface and to orientate permanent dipolescontaining fluorine in side chains of the polymer chains parallel to theelectric field. However, there has been a problem in view of practicaluse that the orientation of permanent dipoles achieved by a polingtreatment tends to relax, because an opposite electric charge of wateror an ion in the air can easily attach to a polarized film surface inthe direction of canceling the orientation, and the piezoelectricitydeclines remarkably with time.

Although PVDF is a material that exhibits the highest piezoelectricityamong the above described polymeric piezoelectric materials, itsdielectric constant is 13 and relatively high among polymericpiezoelectric materials, and therefore the piezoelectric g constant(open circuit voltage per unit stress), which is a value obtained bydividing a piezoelectric d constant by a dielectric constant, becomessmall. In addition, although PVDF exhibits favorable conversionefficiency from electricity to sound, improvement in the conversionefficiency from sound to electricity has been looked for.

In recent years, use of a polymer having optical activity, such aspolypeptide and polylactic acid, has drawn attention in addition to theabove polymeric piezoelectric materials. A polylactic acid-type polymeris known to exhibit piezoelectricity by a simple mechanical stretchingoperation. Among polymers having optical activity, the piezoelectricityof a polymer crystal, such as polylactic acid, results from permanentdipoles of C═O bonds existing in the screw axis direction. Especially,polylactic acid, in which the volume fraction of side chains withrespect to a main chain is small and the content of permanent dipolesper volume is large, thereby constituting a sort of ideal polymer amongpolymers having helical chirality. Polylactic acid exhibitingpiezoelectricity only by a stretching treatment does not require apoling treatment and is known to maintain the piezoelectric moduluswithout decrease for several years.

Since polylactic acid exhibits various piezoelectric properties asdescribed above, various polymeric piezoelectric materials usingpolylactic acid have been reported. For example, a polymericpiezoelectric material exhibiting a piezoelectric modulus ofapproximately 10 pC/N at normal temperature, which is attained by astretching treatment of a molding of polylactic acid, has been disclosed(e.g., see Japanese Patent Application Laid-Open (JP-A) No. H5-152638).Further, it has been also reported that high piezoelectricity ofapproximately 18 pC/N can be achieved by a special orientation methodcalled as a forging process for orientating highly polylactic acidcrystals (e.g., see JP-A-2005-213376).

SUMMARY OF INVENTION Technical Problem

However, since piezoelectric materials (films) described in JP-A-No.H5-152638, and JP-A-No. 2005-213376 are produced by stretchingprincipally uniaxially, they are easily torn parallel to the stretchingdirection, and they have a drawback in that the tear strength in acertain direction is low. The tear strength in a certain direction ishereinafter also referred to as “longitudinal tear strength”.

Further, the transparency of any of the piezoelectric materialsdescribed in JP-A-No. H5-152638, and JP-A-No. 2005-213376 isinsufficient

The present invention has been made in view of the above circumstancesand provides a polymeric piezoelectric material which has a highpiezoelectric constant d₁₄, which is superior in transparency, and inwhich deterioration of the longitudinal tear strength is suppressed; anda process for producing the same.

Solution to Problem

Specific measures to attain the object are as follows.

-   [1] A polymeric piezoelectric material comprising a helical chiral    polymer having a

weight-average molecular weight of from 50,000 to 1,000,000 and havingoptical activity, wherein a crystallinity of the material measured by aDSC method is from 20% to 80%, and a product of a standardized molecularorientation MORc measured by a microwave transmission type molecularorientation meter based on a reference thickness of 50 μm and thecrystallinity is from 25 to 250.

-   [2] The polymeric piezoelectric material according to [1], wherein    the crystallinity is 40.8% or less.-   [3] The polymeric piezoelectric material according to [1] or [2],    wherein an internal haze with respect to visible light is 40% or    less.-   [4] The polymeric piezoelectric material according to any one of [1]    to [3], wherein the standardized molecular orientation MORc is from    1.0 to 15.0.-   [5] The polymeric piezoelectric material according to any one of [1]    to [4], wherein a piezoelectric constant d₁₄ measured by a    displacement method at 25° C. is 1 pm/V or higher.-   [6] The polymeric piezoelectric material according to any one of [1]    to [5], wherein the helical chiral polymer is a polylactic acid-type    polymer having a main chain comprising a repeating unit represented    by the following formula (1):

-   [7] The polymeric piezoelectric material according to any one of [1]    to [6], wherein an optical purity of the helical chiral polymer is    95.00% ee or higher.-   [8] The polymeric piezoelectric material according to any one of [1]    to [7], wherein a content of the helical chiral polymer is 80 mass %    or more.-   [9] The polymeric piezoelectric material according to any one of [1]    to [8], wherein an internal haze with respect to visible light is    1.0% or less.-   [10] A process for producing the polymeric piezoelectric material    according to any one of [1] to [9], comprising a first step of    heating a sheet in an amorphous state containing the helical chiral    polymer to obtain a pre-crystallized sheet, and a second step of    stretching the pre-crystallized sheet simultaneously in biaxial    directions.-   [11] The process for producing the polymeric piezoelectric material    according to [10], wherein, in the first step for obtaining the    pre-crystallized sheet, the sheet in an amorphous state is heated at    a temperature T satisfying the following formula until the    crystallinity becomes between 1% and 70%:

Tg−40° C.≦T≦Tg+40° C.

wherein Tg represents a glass transition temperature of the helicalchiral polymer.

-   [12] The process for producing the polymeric piezoelectric material    according to [10] or [11], wherein, in the first step for obtaining    the pre-crystallized sheet, the sheet in an amorphous state    containing polylactic acid as the helical chiral polymer is heated    at from 20° C. to 170° C. for from 5 sec to 60 min.-   [13] The process for producing the polymeric piezoelectric material    according to any one of [10] to [12], wherein an annealing treatment    is conducted after the second step.

ADVANTAGEOUS EFFECTS OF INVENTION

By virtue of the present invention, a polymeric piezoelectric materialwhich has a high piezoelectric constant d₁₄, which is superior intransparency, and in which deterioration of the longitudinal tearstrength is suppressed; and a process for producing the same can beprovided.

DESCRIPTION OF EMBODIMENTS

A polymeric piezoelectric material according to the present inventioncontains a helical chiral polymer with the weight-average molecularweight from 50,000 to 1,000,000 having optical activity (hereinafteralso referred to as “optically active polymer”), wherein thecrystallinity measured by a DSC method is from 20% to 80%; and theproduct of a standardized molecular orientation MORc measured by amicrowave transmission type molecular orientation meter based on areference thickness of 50 μm and the crystallinity is from 25 to 250.

According to the composition, a polymeric piezoelectric material canhave a high piezoelectric constant d₁₄, be superior in the transparency,and deterioration of the longitudinal tear strength (the tear strengthin a certain direction) therein can be suppressed.

More particularly, a polymeric piezoelectric material with the abovecomposition can suppress a phenomenon of deterioration of thelongitudinal tear strength (the tear strength in a certain direction)while maintaining high piezoelectricity (high piezoelectric constantd₁₄) and high transparency by choosing the crystallinity in a range from20% to 80% and the product of the MORc and the crystallinity from 25 to250.

That the tear strength in a certain direction deteriorates isoccasionally expressed herein as “longitudinal tear strengthdeteriorates”, and a situation where the tear strength in a certaindirection is low, is occasionally expressed herein as “longitudinal tearstrength is low”.

Further, that a phenomenon of deterioration of the tear strength in acertain direction is suppressed, is occasionally expressed herein as“longitudinal tear strength is improved”, and a situation where thephenomenon of deterioration of the tear strength in a certain directionis suppressed, is occasionally expressed as “longitudinal tear strengthis high” or “superior in longitudinal tear strength”.

In the current embodiment, a “piezoelectric constant d₁₄” is a kind oftensor of a piezoelectric modulus and determined from the degree ofpolarization appeared in the direction of shear stress, when the shearstress is applied in the direction of the stretching axis of a stretchedmaterial. Specifically, the appeared electric charge density per unitshear stress is defined as d₁₄. A higher value of the piezoelectricconstant d₁₄ means that piezoelectricity is the higher. An abbreviatedexpression of “piezoelectric constant” means herein a “piezoelectricconstant d₁₄”.

Meanwhile, a piezoelectric constant d₁₄ is a value determined by thefollowing method.

Namely, a rectangular film with a longer direction inclined to 45° fromthe stretching direction is used as a specimen. Electrode layers areformed on the entire surfaces of both sides of the principal plane ofthe specimen. The amount of strain in the longer direction of the filmcaused by impressing the electrodes with application voltage E(V) isregarded as X. Regarding the quotient of application voltage E(V)divided by film thickness t (m) as electric field strength E (V/m), andthe amount of strain in the longer direction of the film caused by theapplication voltage E(V) as X, d₁₄ is a value defined as 2×amount ofstrain X/electric field strength E (V/m).

A complex piezoelectric modulus d₁₄ is calculated as d₁₄=d₁₄′−id₁₄″,wherein d₁₄′ and d₁₄″ are obtained by Rheolograph-Solid, Model S-1 (byToyo Seiki Seisaku-Sho, Ltd.). d₁₄′ represents the real part of acomplex piezoelectric modulus, id₁₄″ represents the imaginary part of acomplex piezoelectric modulus, and d₁₄′ (the real part of the complexpiezoelectric modulus) corresponds to the piezoelectric constant d₁₄ ofthe current embodiment. A higher value of the real part of a complexpiezoelectric modulus means that the piezoelectricity is the better.

There are a piezoelectric constant d₁₄ measured by a displacement method(unit: pm/V) and the same measured by a resonance method (unit: pC/N).

[Helical Chiral Polymer Having Optical Activity]

A helical chiral polymer having optical activity refers to a polymerhaving a helical molecular structure and having molecular opticalactivity.

Hereinafter, a helical chiral polymer with the weight-average molecularweight from 50,000 to 1,000,000 having optical activity is also referredto as an “optically active polymer”.

Examples of the optically active polymer include polypeptide, cellulose,a cellulose derivative, a polylactic acid-type resin, polypropyleneoxide, and poly(β-hydroxybutyric acid). Examples of the polypeptideinclude poly(γ-benzyl glutarate), and poly(γ-methyl glutarate). Examplesof the cellulose derivative include cellulose acetate, and cyanoethylcellulose.

The optical purity of the optically active polymer is preferably 95.00%ee or higher, more preferably 99.00% ee or higher, and furtherpreferably 99.99% ee or higher from a viewpoint of enhancing thepiezoelectricity of a polymeric piezoelectric material. Ideally it is100.00% ee. It is presumed that, by selecting the optical purity of theoptically active polymer in the above range, packing of a polymercrystal exhibiting piezoelectricity becomes denser and as the result thepiezoelectricity is improved.

The optical purity of the optically active polymer in the currentembodiment is a value calculated according to the following formula:

Optical purity (% ee)=100×|L-form amount−D-form amount|/(L-formamount+D-form amount);

namely the value of “the difference (absolute value) between L-formamount [mass-%] of the optically active polymer and D-form amount[mass-%] of the optically active polymer” divided by “the total ofL-form amount [mass-%] of the optically active polymer and D-form amount[mass-%] of the optically active polymer” multiplied by “100” is definedas optical purity.

In this regard, for the L-form amount [mass-%] of the optically activepolymer and the D-form amount [mass-%] of the optically active polymer,values to be obtained by a method using high performance liquidchromatography (HPLC) are used. Specific particulars with respect to ameasurement will be described below.

Among the above optically active polymers, a compound with the mainchain containing a repeating unit according to the following formula (1)is preferable from a viewpoint of enhancement of the optical purity andimproving the piezoelectricity.

As an example of a compound with the main chain containing a repeatingunit according to the formula (1) is named a polylactic acid-type resin.Among others, polylactic acid is preferable, and a homopolymer ofL-lactic acid (PLLA) or a homopolymer of D-lactic acid (PDLA) is mostpreferable.

The polylactic acid-type resin means “polylactic acid”, a “copolymer ofone of L-lactic acid or D-lactic acid, and a copolymerizablemulti-functional compound”, or a mixture of the two. The “polylacticacid” is a polymer linking lactic acid by polymerization through esterbonds into a long chain, and it is known that polylactic acid can beproduced by a lactide process via a lactide, a direct polymerizationprocess, by which lactic acid is heated in a solvent under a reducedpressure for polymerizing while removing water, or the like. Examples ofthe “poly(lactic acid)” include a homopolymer of L-lactic acid, ahomopolymer of D-lactic acid, a block copolymer including a polymer ofat least one of L-lactic acid and D-lactic acid, and a graft copolymerincluding a polymer of at least one of L-lactic acid and D-lactic acid.

Examples of the “copolymerizable multi-functional compound” include ahydroxycarboxylic acid, such as glycolic acid, dimethylglycolic acid,3-hydroxybutyric acid, 4-hydroxybutyric acid, 2-hydroxypropanoic acid,3-hydroxypropannoic acid, 2-hydroxyvaleric acid, 3-hydroxyvaleric acid,4-hydroxyvaleric acid, 5-hydroxyvaleric acid, 2-hydroxycaproic acid,3-hydroxycaproic acid, 4-hydroxycaproic acid, 5-hydroxycaproic acid,6-hydroxycaproic acid, 6-hydroxymethylcaproic acid, and mandelic acid; acyclic ester, such as glycolide, β-methyl-δ-valerolactone,γ-valerolactone, and ε-caprolactone; a polycarboxylic acid, such asoxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, azelaic acid, sebacic acid, undecanedioic acid,dodecanedioic acid, and terephthalic acid, and an anhydride thereof; apolyhydric alcohol, such as ethylene glycol, diethylene glycol,triethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol,1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,9-nonanediol, 3-methyl-1,5-pentanediol, neopentylglycol,tetramethylene glycol, and 1,4-hexanedimethanol; a polysaccharide suchas cellulose; and an aminocarboxylic acid such as α-amino acid.

Examples of the “copolymer of one of L-lactic acid or D-lactic acid, anda copolymerizable multi-functional compound” include a block copolymeror a graft copolymer having a polylactic acid sequence, which can form ahelical crystal.

The concentration of a structure derived from a copolymer component inthe optically active polymer is preferably 20 mol % or less. Forexample, if the optically active polymer is a polylactic acid-typepolymer, with respect to the total number of moles of a structurederived from lactic acid and a structure derived from a compoundcopolymerizable with lactic acid (copolymer component) in the polylacticacid-type polymer, the copolymer component is preferably 20 mol % orless.

The optically active polymer (for example, polylactic acid-type resin)can be produced, for example, by a process for obtaining the polymer bydirect dehydration condensation of lactic acid, as described inJP-A-S59-096123 and JP-A-H7-033861, or a process for obtaining the sameby a ring-opening polymerization of lactide, which is a cyclic dimer oflactic acid, as described in U.S. Pat. No. 2,668,182 and No. 4,057,357.

In order to make the optical purity of the optically active polymer (forexample, polylactic acid-type resin) obtained by any of the productionprocesses to 95.00% ee or higher, for example, if a polylactic acid isproduced by a lactide process, it is preferable to polymerize lactide,whose optical purity has been enhanced to 95.00% ee or higher by acrystallization operation.

[Weight-Average Molecular Weight of Optically Active Polymer]

The weight-average molecular weight (Mw) of the optically active polymeraccording to the current embodiment is from 50,000 to 1,000,000. If thelower limit of the weight-average molecular weight of the opticallyactive polymer is less than 50,000, the mechanical strength of a moldingfrom the optically active polymer becomes insufficient. The lower limitof the weight-average molecular weight of the optically active polymeris preferably 100,000 or higher, and more preferably 150,000 or higher.Meanwhile, if the upper limit of the weight-average molecular weight ofthe optically active polymer exceeds 1,000,000, molding of the opticallyactive polymer (for example, by extrusion molding, etc. to a film form,etc.) becomes difficult.

The upper limit of the weight-average molecular weight is preferably800,000 or less, and more preferably 300,000 or less.

Further, the molecular weight distribution (Mw/Mn) of the opticallyactive polymer is preferably from 1.1 to 5, more preferably from 1.2 to4, and further preferably from 1.4 to 3, from a viewpoint of thestrength of a polymeric piezoelectric material. The weight-averagemolecular weight Mw and the molecular weight distribution (Mw/Mn) of apolylactic acid-type polymer are measured using a gel permeationchromatograph (GPC) by the following GPC measuring method.

GPC Measuring Apparatus:

GPC-100 by Waters Corp.

Column:

SHODEX LF-804 by Showa Denko K.K.

Preparation of Sample:

A polylactic acid-type polymer is dissolved in a solvent (e.g.chloroform) at 40° C. to prepare a sample solution with theconcentration of 1 mg/mL.

Measurement Condition:

A sample solution 0.1 mL is introduced into the column at a temperatureof 40° C. and a flow rate of 1 mL/min by using chloroform as a solvent.

The sample concentration in a sample solution separated by the column ismeasured by a differential refractometer. Based on polystyrene standardsamples, a universal calibration curve is created and the weight-averagemolecular weight (Mw) and the molecular weight distribution (Mw/Mn) of apolylactic acid-type polymer are calculated.

For a polylactic acid-type polymer, a commercial polylactic acid may beused, and examples thereof include PURASORB (PD, PL) by Purac Corporate,LACEA (H-100, H-400) by Mitsui Chemicals, Inc., and Ingeo 4032D, 4043Dby NatureWorks LLC.

If a polylactic acid-type resin is used as the optically active polymerand in order to make the weight-average molecular weight (Mw) of thepolylactic acid-type resin 50,000 or higher, it is preferable to producethe optically active polymer by a lactide process, or a directpolymerization process.

A polymeric piezoelectric material of the current embodiment may containonly one kind of the optically active polymer, or may contain two ormore kinds thereof.

Although there is no particular restriction on a content of theoptically active polymer (if two or more kinds are used, the totalcontent; hereinafter holds the same) in a polymeric piezoelectricmaterial of the current embodiment, 80 mass % or more with respect tothe total mass of the polymeric piezoelectric material is preferable.

If the content is 80 mass % or more, the piezoelectric constant tends toimprove.

[Other Components]

A polymeric piezoelectric material of the current embodiment maycontain, to the extent that the advantage of the current embodiment benot compromised, components other than the optically active polymers(for example, publicly known resins, as represented by polyvinylidenefluoride, a polyethylene resin and a polystyrene resin, inorganicfillers, such as silica, hydroxyapatite, and montmorillonite, andpublicly known crystal nucleating agents such as phthalocyanine).

Further, from a viewpoint of better inhibition of a structural change byhydrolysis, etc. a polymeric piezoelectric material of the currentembodiment should preferably contain a stabilizing agent such as acarbodiimide compound as represented by CARBODILITE (registered trademark).

Further, to the extent that the advantage of the current embodiment isnot compromised, a polymeric piezoelectric material of the currentembodiment may contain a helical chiral polymer other than theafore-described optically active polymer (namely, a helical chiralpolymer having a weight-average molecular weight (Mw) from 50,000 to1,000,000 and having optical activity).

Inorganic Filler

A polymeric piezoelectric material of the current embodiment may containat least one kind of inorganic filler.

For example, in order to form a polymeric piezoelectric material to atransparent film inhibiting generation of a void such as an air bubble,an inorganic filler such as hydroxyapatite may be dispersed into apolymeric piezoelectric material in a nano-state. However for dispersingthe inorganic filler into a nano-state, large energy is required todisintegrate an aggregate, and if the inorganic filler is not dispersedin a nano-state the film transparency may occasionally be compromised.Therefore, if a polymeric piezoelectric material according to thecurrent embodiment contains an inorganic filler, the content of theinorganic filler with respect to the total mass of the polymericpiezoelectric material is preferably less than 1 mass-%.

Further, if a polymeric piezoelectric material contains components otherthan the optically active polymer, the content of the components otherthan the optically active polymer is preferably 20 mass % or less, andmore preferably 10 mass % or less with respect to the total mass of thepolymeric piezoelectric material.

Crystallization Accelerator (Crystal Nucleating Agent)

A polymeric piezoelectric material of the current embodiment may containat least one kind of crystallization accelerator (crystal nucleatingagent).

Although there is no particular restriction on a crystallizationaccelerator (crystal nucleating agent) insofar as a crystallizationaccelerating effect can be recognized, it is preferable to select asubstance with the crystal structure having lattice spacing close to thelattice spacing of the crystal lattice of the optically active polymer.This is because a substance having closer lattice spacing has the higheractivity as a nucleating agent. For example, when the polylacticacid-type resin is used as the optically active polymer, examplesinclude an organic substance, such as zinc phenylsulfonate, melaminepolyphosphate, melamine cyanurate, zinc phenylphosphonate, calciumphenylphosphonate, and magnesium phenylphosphonate, and an inorganicsubstance, such as talc and clay. Among others, zinc phenylphosphonate,which has lattice spacing closest to the lattice spacing of polylacticacid and exhibits excellent crystallization accelerating activity, ispreferable. Meanwhile, a commercial product can be used as acrystallization accelerator. Specific examples thereof includeECOPROMOTE (zinc phenylphosphonate: by Nissan Chemical Ind., Ltd.).

The content of a crystal nucleating agent with respect to 100 parts bymass of the optically active polymer is normally from 0.01 to 1.0 partby mass, preferably from 0.01 to 0.5 parts by mass, and from a viewpointof better crystallization accelerating activity and maintenance of abiomass ratio especially preferably from 0.02 to 0.2 parts by mass.

If the content of a crystal nucleating agent is 0.01 parts by mass ormore, the crystallization accelerating effect can be attained moreeffectively. If the content of a crystal nucleating agent is less than1.0 part, the crystallization speed can be regulated more easily.

From a viewpoint of transparency, the polymeric piezoelectric materialshould preferably not contain a component other than a helical chiralpolymer having optical activity.

[Structure]

In a polymeric piezoelectric material of the current embodiment,optically active polymers are orientated.

Molecular orientation ratio MOR is used as an index representing theorientation. Namely, the molecular orientation ratio (MOR) is a valueindicating the degree of molecular orientation, and measured by thefollowing microwave measurement method. Namely, a sample (film) isplaced in a microwave resonant waveguide of a well known microwavemolecular orientation ratio measuring apparatus (also referred to as a“microwave transmission-type molecular orientation meter”) such that thesample plane (film plane) is arranged perpendicular to the travellingdirection of the microwaves. Then, the sample is continuously irradiatedwith microwaves whose oscillating direction is biased unidirectionally,while maintaining such conditions, the sample is rotated in a planeperpendicular to the travelling direction of the microwaves from 0 to360°, and the intensity of the microwaves passed through the sample ismeasured to determine the molecular orientation ratio MOR.

Standardized molecular orientation MORc in the current embodiment meansa MOR value to be obtained at the reference thickness tc of 50 μm, andcan be determined by the following formula.

MORc=(tc/t)×(MOR−1)+1

(tc: Reference thickness corrected to; t: Sample thickness)

A standardized molecular orientation MORc can be measured by a publiclyknown molecular orientation meter, e.g. a microwave-type molecularorientation analyzer MOA-2012A or MOA-6000 by Oji ScientificInstruments, at a resonance frequency in the vicinity of 4 GHz or 12GHz.

The standardized molecular orientation MORc can be regulated bycrystallization conditions (for example, heating temperature and heatingtime) and stretching conditions (for example, stretching temperature andstretching speed) in producing a polymeric piezoelectric material.

Standardized molecular orientation MORc can be converted tobirefringence Δn, which equals to retardation divided by film thickness.

More specifically, the retardation can be measured by RETS100, by OtsukaElectronics Co., Ltd. Further, MORc and Δn are approximately in alinearly proportional relationship, and if Δn is 0, MORc is 1.

<Physical Properties of Polymeric Piezoelectric Material>

A polymeric piezoelectric material of the current embodiment has a highpiezoelectric constant (a piezoelectric constant d₁₄ measured by adisplacement method at 25° C. is preferably 1 pm/V or higher) and issuperior in transparency, and longitudinal tear strength.

[Piezoelectric Constant (Displacement Method)]

The piezoelectric constant of a polymeric piezoelectric material in thecurrent embodiment is a value measured as follows.

Firstly, a polymeric piezoelectric material is cut into a size of 40 mmin the stretching direction (MD direction) and 40 mm in the directionperpendicular to the stretching direction (TD direction) respectively toprepare a rectangular specimen. Then the prepared specimen is set on astage of a sputtering thin-film formation apparatus JSP-8000 by UlvacInc., and the inside of a coater chamber is evacuated to a vacuum stateby a rotary pump (for example, 10⁻³ Pa or less). Then a sputteringtreatment is conducted for 500 sec on a plane of the specimen under theconditions of an Ag (silver) target at an applied voltage of 280 V and asputtering current of 0.4 A. Then a sputtering treatment is carried outon the other plane under the same conditions for 500 sec, therebyforming Ag coats on both the planes of the specimen and completing Agconductive layers.

A specimen (a polymeric piezoelectric material) of 40 mm×40 mm with theAg conductive layers on both planes thus formed is cut to a length of 32mm in the direction of 45° with respect to the stretching direction (MDdirection) of a polymeric piezoelectric material, and to 5 mm in thedirection perpendicular to the above 45° direction, to cut out a pieceof rectangular film in a size of 32 mm×5 mm. This is used as a samplefor measuring the piezoelectric constant.

A sine-wave alternating voltage of 10 Hz and 300 Vpp is applied to theprepared sample, and a difference distance between the maximum value andthe minimum value of the displacement of the film is measured by a laserspectral-interference displacement meter SI-1000 (by KeyenceCorporation).

A value obtained by dividing the measured displacement (mp-p) by thereference length of the film, which is 30 mm, is defined as strain, anda value obtained by dividing the strain by an electric field intensityapplied to the film ((applied voltage (V))/(film thickness)) andmultiplying by 2, is defined as piezoelectric constant d₁₄.

A higher piezoelectric constant results in the larger displacement of apolymeric piezoelectric material responding to a voltage applied to thematerial, and reversely the higher voltage generated responding to aforce applied to a polymeric piezoelectric material, and therefore isadvantageous as a polymeric piezoelectric material.

Specifically, the piezoelectric constant d₁₄ measured by a displacementmethod at 25° C. is preferably 1 pm/V or higher, more preferably 3 pm/Vor higher, further preferably 4 pm/V or higher. Although there is noparticular upper limit of the piezoelectric constant, it is preferably50 pm/V or less, and sometimes more preferably 30 pm/V or less, for apiezoelectric material using a helical chiral polymer from a viewpointof the balance with transparency, etc. as described below.

Similarly, from a viewpoint of the balance with transparency, thepiezoelectric constant d₁₄ measured by a resonance method is preferably15 pC/N or less.

“MD direction” means herein a flow direction of a film (MachineDirection), and “TD direction” means a direction orthogonal to the MDdirection and parallel to the principal plane of the film (TransverseDirection).

[Crystallinity]

The crystallinity of a polymeric piezoelectric material is determined bya DSC method, and the crystallinity of a polymeric piezoelectricmaterial of the current embodiment is from 20% to 80%, and preferablyfrom 30% to 70%. If the crystallinity is within the range, the balanceamong the piezoelectricity, the transparency and the longitudinal tearstrength of a polymeric piezoelectric material may be favorable, andwhitening or breakage is less likely to occur in stretching thepolymeric piezoelectric material, and therefore production is easy.

Specifically, if the crystallinity is less than 20%, thepiezoelectricity tends to deteriorate.

If the crystallinity exceeds 80%, the longitudinal tear strength and thetransparency tend to deteriorate.

From a viewpoint of improving further the longitudinal tear strength andthe transparency, the crystallinity is preferably 40.8% or less andespecially 40.0% or less.

In the current embodiment the crystallinity of a polymeric piezoelectricmaterial can be regulated within a range from 20% to 80% by regulating,for example, conditions for crystallization and stretching in producinga polymeric piezoelectric material.

[Transparency (Internal Haze)]

The transparency of a polymeric piezoelectric material can be evaluated,for example, by visual observation or haze measurement.

The internal haze of a polymeric piezoelectric material with respect tovisible light is preferably 40% or less. In this regard, the internalhaze is a value measured for a polymeric piezoelectric material with thethickness from 0.03 mm to 0.05 mm using a haze meter (TC-HIII DPK, byTokyo Denshoku Co., Ltd.) at 25° C. according to JIS-K7105, and detailsof the measuring method are described in Examples below.

The internal haze of a polymeric piezoelectric material is preferablynot more than 20% and more preferably not more than 5%. Further, from aviewpoint of improvement of longitudinal tear strength, the internalhaze of a polymeric piezoelectric material is preferably not more than2.0% and especially preferably not more than 1.0%.

Further, the lower the internal haze, the better a polymericpiezoelectric material is. However, from a viewpoint of the balance withthe piezoelectric constant, etc. the internal haze is preferably from0.0% to 40%, more preferably from 0.01% to 20%, further preferably from0.01% to 5%, further preferably from 0.01% to 2.0%, and especiallypreferably from 0.01% to 1.0%.

Incidentally, “internal haze” means herein the internal haze of apolymeric piezoelectric material according to the present invention. Theinternal haze is a haze from which a haze caused by the shape of anexternal surface of the polymeric piezoelectric material is excluded, asdescribed in an Example below.

[Standardized Molecular Orientation MORc]

The standardized molecular orientation MORc of a polymeric piezoelectricmaterial of the current embodiment is preferably from 1.0 to 15.0, andmore preferably from 4.0 to 10.0.

If the standardized molecular orientation MORc is 1.0 or more, a largenumber of molecular chains of the optically active polymer (for example,poly(lactic acid) molecular chains) are aligned in the stretchingdirection, and as the result a higher rate of generation of orientedcrystals can be attained to exhibit higher piezoelectricity.

If the standardized molecular orientation MORc is 15.0 or less, thelongitudinal tear strength can be improved.

[Product of Standardized Molecular Orientation MORc and Crystallinity]

The product of the crystallinity and the standardized molecularorientation MORc of a polymeric piezoelectric material in the currentembodiment is from 25 to 250. By regulation within the range, highpiezoelectricity and high transparency can be maintained anddeterioration of the longitudinal tear strength (namely, tear strengthin a certain direction) can be suppressed.

If the product of the crystallinity and the standardized molecularorientation MORc of a polymeric piezoelectric material is less than 25,the piezoelectricity tends to deteriorate.

If the product of the crystallinity and the standardized molecularorientation MORc of a polymeric piezoelectric material exceeds 250, thelongitudinal tear strength and the transparency tend to deteriorate.

The product of the crystallinity and the MORc is more preferably from 50to 200, and further preferably from 100 to 190.

In the current embodiment the product of the crystallinity and thestandardized molecular orientation MORc of a polymeric piezoelectricmaterial can be regulated within a range from 25 to 250 by regulatingconditions for crystallization and stretching in producing a polymericpiezoelectric material.

[Longitudinal Tear Strength]

The longitudinal tear strength of a polymeric piezoelectric material ofthe current embodiment is evaluated based on the tear strength measuredaccording to the “Right angled tear method” stipulated in JIS K 7128-3“Plastics—Tear strength of films and sheets”.

In this regard, the crosshead speed of a tensile testing machine is setat 200 m/min and tear strength is calculated according to the followingformula:

T=F/d

wherein T stands for the tear strength (N/mm), F for the maximum tearload, and d for the thickness (mm) of a specimen.

[Dimensional Stability]

It is preferable that the dimensional change rate of a polymericpiezoelectric material under heat is low, especially at a temperature ofan environment where devices or apparatus described below, such as aloudspeaker and a touch panel, incorporating the material are used.Because, if the dimension of a piezoelectric material changes in aservice environment of a device, positions of wiring, etc. connectedwith the piezoelectric material are moved, which may causemalfunctioning of the device. The dimensional stability of a polymericpiezoelectric material is evaluated by a dimensional change rate beforeand after a heat treatment for 10 min at 150° C., which is a temperatureslightly higher than the service environment of a device as describedbelow. The dimensional change rate is preferably 10% or less, and morepreferably 5% or less.

[Production of Polymeric Piezoelectric Material]

There is no particular restriction on a process for producing apolymeric piezoelectric material according to the present invention,insofar as the crystallinity can be regulated from 20% to 80% and theproduct of the standardized molecular orientation MORc and thecrystallinity can be regulated from 25 to 250.

As such a process, a process, by which a sheet in an amorphous statecontaining the optically active polymer as described above is subjectedto crystallization and stretching (either may be earlier) can beapplied, and by regulating the respective conditions for thecrystallization and the stretching the crystallinity can be regulatedfrom 20% to 80% and the product of the standardized molecularorientation MORc and the crystallinity can be regulated from 25 to 250,can be used.

The term “crystallization” herein is a concept includingpre-crystallization described below and an annealing treatment describedbelow.

A sheet in an amorphous state means a sheet obtained by heating a simpleoptically active polymer or a mixture containing the optically activepolymer to a temperature equal to or above the melting point Tm of theoptically active polymer and then quenching the same. The quenchingtemperature is for example 50° C.

In a process for producing a polymeric piezoelectric material accordingto the present invention, the optically active polymer (polylacticacid-type polymer, etc.) may be used singly, or a mixture of two or moreoptically active polymers (polylactic acid-type polymers, etc.)described above or a mixture of at least one optically active polymerdescribed above and at least one other component may be used as a rawmaterial for a polymeric piezoelectric material (or a sheet in anamorphous state).

The mixture is preferably a mixture obtained by melt-kneading.

Specifically, when two or more optically active polymers are mixed, orat least one optically active polymer and another component (forexample, the inorganic filler and the crystal nucleating agent) aremixed, optically active polymer(s) to be mixed (according to need,together with another component) are melt-kneaded in a melt-kneadingmachine (LABO PLASTOMILL mixer, by Toyo Seiki Seisaku-sho, Ltd.) underconditions of the mixer rotating speed of from 30 rpm to 70 rpm at from180° C. to 250° C. for from 5 min to 20 min to obtain a blend of pluralkinds of optically active polymers or a blend of an optically activepolymer and another component such as an inorganic filler.

Embodiments of a process for producing a polymeric piezoelectricmaterial according to the present invention will be described below,provided that a process for producing a polymeric piezoelectric materialaccording to the present invention is not limited to the followingembodiments.

First Embodiment

The first embodiment of a process for producing a polymericpiezoelectric material according to the present invention includes, forexample, a first step of heating a sheet in an amorphous statecontaining the optically active polymer (namely, a helical chiralpolymer with the weight-average molecular weight from 50,000 to1,000,000 having optical activity) to obtain a pre-crystallized sheet,and a second step of stretching the pre-crystallized sheet in biaxialdirections (for example, while stretching mainly in a uniaxialdirection, simultaneously or successively stretched in a directiondifferent from said stretching direction).

Generally, by intensifying a force applied to a film during stretching,there appears tendency that the orientation of optically active polymersis promoted, and the piezoelectric constant is enhanced, meanwhile,crystallization is progressed to increase the crystal size, andconsequently the internal haze also increases. Further, as the result ofincrease in internal stress, the rate of dimensional change tends toincrease. If a force is applied simply to a film, not oriented crystals,such as spherocrystals, are formed. Poorly oriented crystals such asspherocrystals increase the internal haze but hardly contribute toincrease in the piezoelectric constant.

Therefore to produce a film having a high piezoelectric constant, andlow internal haze, it is preferable to form efficiently such micro-sizedorientated crystals, as contribute to the piezoelectric constant but notincrease the internal haze.

From the above, for example, by producing a pre-crystallized sheethaving minute crystals (crystallites) formed by pre-crystallization in asheet prior to stretching and then stretching the pre-crystallizedsheet, a force of stretching can be acted efficiently on alow-crystallinity polymer part between a crystallite and a crystalliteinside the pre-crystallized sheet. By this, the optically active polymercan be oriented efficiently in a principal stretching direction.

Specifically, by stretching the pre-crystallized sheet, minuteorientated crystals are formed in a low-crystallinity polymer partbetween a crystallite and a crystallite and at the same timespherocrystals formed by pre-crystallization are collapsed and lamellarcrystals constituting the spherocrystals are aligned as tied in a row bytie-molecular chains in the stretching direction. By this, a desiredMORc value can be attained.

As the result, by stretching the pre-crystallized sheet, a sheet with alow internal haze value can be obtained without compromising remarkablythe piezoelectric constant. Further, by regulating the productionconditions a polymeric piezoelectric material superior in dimensionalstability can be obtained.

However, according to the method of stretching a pre-crystallized sheet,since polymer chains in a low-crystallinity part inside apre-crystallized sheet are disentangled and aligned in a stretchingdirection by stretching, the tear strength against a force from adirection nearly orthogonal to the stretching direction is improved, butreversely the tear strength against a force from a direction nearlyparallel to the stretching direction may be deteriorated.

In view of the above, the constitution of the first embodiment includesa first step of heating a sheet in an amorphous state containing theoptically active polymer to obtain a pre-crystallized sheet, and asecond step of stretching the pre-crystallized sheet in biaxialdirections.

In the first embodiment, when the pre-crystallized sheet is stretched inthe second step (stretching step) in order to improve thepiezoelectricity (also referred to as “principal stretching”), thepre-crystallized sheet is stretched simultaneously or successively in adirection crossing the stretching direction of the principal stretching(also referred to as “secondary stretching”) to perform biaxialstretching. This can align molecular chains in the sheet not only in thedirection of the principal stretching axis but also in the directioncrossing the principal stretching axis, and as a consequence the productof the standardized molecular orientation MORc and the crystallinity canbe regulated appropriately within a certain range (specifically from 25to 250).

As the result, while maintaining the transparency, the piezoelectricitycan be enhanced, and further the longitudinal tear strength can beimproved.

For the control of standardized molecular orientation MORc, it isimportant to regulate the heating time and the heating temperature for asheet in an amorphous state in the first step, and the stretching speedand the stretching temperature for a pre-crystallized sheet in thesecond step.

As described above, the optically active polymer is a polymer having ahelical molecular structure and having molecular optical activity.

A sheet in an amorphous state containing the optically active polymermay be those commercially available, or produced by a publicly knownfilm forming process such as an extrusion process. A sheet in anamorphous state may have s single layer or multiple layers.

[First Step (Pre-Crystallization Step)]

The first step in the first embodiment is a step of heating a sheet inan amorphous state containing the optically active polymer to obtain apre-crystallized sheet.

As a treatment through the first step and the second step in the firstembodiment, specifically, it may be: 1) a treatment (off-line treatment)by which a sheet in an amorphous state is heat-treated to apre-crystallized sheet (up to here the first step), and the obtainedpre-crystallized sheet is set in a stretching apparatus and stretched(up to here the second step); or 2) a treatment (in-line treatment) bywhich a sheet in an amorphous state is set in a stretching apparatus,and heated in the stretching apparatus to a pre-crystallized sheet (upto here the first step), and the obtained pre-crystallized sheet iscontinuously stretched in the stretching apparatus (up to here thesecond step).

Although there is no particular restriction on a heating temperature Tfor pre-crystallizing a sheet in an amorphous state containing theoptically active polymer in the first step, from viewpoints of enhancingthe piezoelectricity, the transparency, etc. of a polymericpiezoelectric material produced, it should be preferably a temperatureset to satisfy the following relational expression with respect to theglass transition temperature Tg of the optically active polymer, and tomake the crystallinity from 1% to 70%.

Tg−40° C.≦T≦Tg+40° C.

(Tg stands for the glass transition temperature of the optically activepolymer.)

The glass transition temperature Tg [° C.] of the optically activepolymer and the melting point Tm [° C.] of the optically active polymerare respectively a glass transition temperature (Tg) obtained as aninflection point of a curve and a temperature (Tm) recognized as a peakvalue of an endothermic reaction, from a melting endothermic curveobtained for the optically active polymer using the differentialscanning calorimeter (DSC) by raising the temperature under a conditionof the temperature increase rate of 10° C./min.

The heat treatment time for pre-crystallization in the first step may beso regulated as to satisfy the crystallinity as desired and to make theproduct of the standardized molecular orientation MORc of a polymericpiezoelectric material after the stretching (after the second step) andthe crystallinity of the polymeric piezoelectric material after thestretching from 25 to 250, preferably from 50 to 200, and furtherpreferably from 100 to 190. If the heat treatment time becomes longer,the crystallinity after the stretching becomes higher and thestandardized molecular orientation MORc after the stretching becomesalso higher. If the heat treatment time becomes shorter, thecrystallinity after the stretching becomes also lower and thestandardized molecular orientation MORc after the stretching becomesalso lower.

If the crystallinity of a pre-crystallized sheet before stretchingbecomes high, conceivably the sheet becomes stiff and a largerstretching stress is exerted on the sheet, and therefore such parts ofthe sheet, where the crystallinity is relatively low, are alsoorientated highly to enhance also the standardized molecular orientationMORc after stretching. Reversely, conceivably, if the crystallinity of apre-crystallized sheet before stretching becomes low, the sheet becomessoft and a stretching stress is exerted to a lesser extent on the sheet,and therefore such parts of the sheet, where the crystallinity isrelatively low, are also orientated weakly to lower also thestandardized molecular orientation MORc after stretching.

The heat treatment time varies depending on the heat treatmenttemperature, the sheet thickness, the molecular weight of a resinconstituting a sheet, and the kind and quantity of an additive. If asheet in an amorphous state is preheated at a temperature allowing thesheet to crystallize on the occasion of preheating which may be carriedout before a stretching step (second step) described below, the actualheat treatment time for crystallizing the sheet corresponds to the sumof the above preheating time and the heat treatment time at thepre-crystallization step before the preheating.

The heat treatment time for a sheet in an amorphous state is preferablyfrom 5 sec to 60 min, and from a viewpoint of stabilization ofproduction conditions more preferably from 1 min to 30 min. If, forexample, a sheet in an amorphous state containing a polylactic acidresin as the optically active polymer is pre-crystallized, heating atfrom 20° C. to 170° C. for from 5 sec to 60 min (preferably from 1 minto 30 min) is preferable.

In the first embodiment, for imparting efficiently piezoelectricity,transparency, and longitudinal tear strength to a sheet afterstretching, it is preferable to adjust the crystallinity of apre-crystallized sheet before stretching.

The reason behind the improvement of the piezoelectricity, etc. bystretching is believed to be because stress by stretching isconcentrated on parts of a pre-crystallized sheet where thecrystallinity is relatively high, which are presumably in a state ofspherocrystal, such that spherocrystals are destroyed and aligned toenhance the piezoelectricity (piezoelectric constant d₁₄); and because,at the same time, the stretching stress is also exerted on parts wherethe crystallinity is relatively low through the spherocrystals, suchthat an orientation of the low crystallinity parts is promoted toenhance the piezoelectricity (piezoelectric constant d₁₄).

The crystallinity of a sheet after stretching is set to aim at 20% to80%, preferably at 30% to 70%. Consequently, the crystallinity of apre-crystallized sheet just before stretching is set at 1% to 70%,preferably at 2% to 60%. The crystallinity of a pre-crystallized sheetmay be carried out similarly as the measurement of the crystallinity ofa polymeric piezoelectric material of the current embodiment afterstretching.

The thickness of a pre-crystallized sheet is selected mainly accordingto an intended thickness of a polymeric piezoelectric material to beattained by means of stretching at the second step and the stretchingratio, and is preferably from 50 μm to 1000 μm, and more preferablyabout from 200 μm to 800 μm.

[Second Step (Stretching Step)]

There is no particular restriction on a stretching process at the secondstep (stretching step), a process combining stretching for formingoriented crystals (also called as principal stretching) and stretchingconducted in a direction crossing the former stretching direction. Bystretching a polymeric piezoelectric material, a polymeric piezoelectricmaterial having a large area principal plane can be also obtained.

In this regard, a “principal plane” means among surfaces of a polymericpiezoelectric material a plane with the largest area. A polymericpiezoelectric material according to the present invention may have twoor more principal planes. For example, if a polymeric piezoelectricmaterial is a platy body having two planes each of rectangular planes Aof 10 mm×0.3 mm, rectangular planes B of 3 mm×0.3 mm, and rectangularplanes C of 10 mm×3 mm, the principal plane of the polymericpiezoelectric material is planes C, and there are 2 principal planes.

As for the principal plane area in the current embodiment, the principalplane area of a polymeric piezoelectric material is preferably 5 mm² ormore, and more preferably 10 mm² or more.

It is presumed that molecular chains of a polylactic acid-type polymercontained in a polymeric piezoelectric material can be orientateduniaxially and aligned densely to attain higher piezoelectricity, if apolymeric piezoelectric material is stretched mainly uniaxially.

Meanwhile, as described above, if stretched only in one direction,polymer molecular chains in a sheet are aligned mainly in the stretchingdirection, and therefore the longitudinal tear strength against a forcefrom a direction nearly orthogonal to the stretching direction maydeteriorate.

When stretching for increasing the piezoelectricity (also referred to as“principal stretching”) is conducted at the stretching step, bystretching a pre-crystallized sheet simultaneously or successively in adirection crossing the stretching direction of the principal stretching(also referred to as “secondary stretching”) for performing biaxialstretching, a polymeric piezoelectric material enjoying excellentbalance of piezoelectricity, transparency, and longitudinal tearstrength can be obtained.

In this regard, “successive stretching” means a stretching process, bywhich a sheet is first stretched in a uniaxial direction, and thenstretched in a direction crossing the first stretching direction.

There is no particular restriction on a process for biaxial stretchingin the second step, and a usual process can be applied. Specifically, acombined process of a roll stretching (stretching in the MD direction)and a tenter stretching (stretching in the TD direction) is preferable.In this case, from a viewpoint of production efficiency, the TDdirection should preferably be selected for the direction with a higherstretching ratio (for example, principal stretching direction), and theMD direction should preferably be selected for the direction with alower stretching ratio (for example, secondary stretching direction).

The biaxial stretching may be conducted simultaneously or successively,however it should preferably be conducted simultaneously (namely,simultaneous biaxial stretching).

A reason for simultaneous biaxial stretching being preferable is that,in a case of successive stretching, at the second or later stretching, aforce crossing the stretching direction at the first stretching isadded, therefore a longitudinal tearing of film may occur during thestretching.

Similarly in a case of successive stretching, from a viewpoint ofsuppression of the longitudinal tearing of film during the second orlater stretching, the ratio of the first stretching should preferably besmall.

In this connection, as described above, “MD direction” means the flowdirection of a film, and “TD direction” means a direction orthogonal tothe MD direction and parallel to the principal plane of the film.

Although there is no particular restriction on the stretching ratio,insofar as the crystallinity of a polymeric piezoelectric material andthe product of the MORc and the crystallinity after the stretching (orafter the annealing treatment, if the annealing step described below isexercised) can be regulated within the above described range, thestretching ratio of the principal stretching is preferably from 2 to8-fold, more preferably from 2.5 to 5-fold, and especially preferablyfrom 2.7 to 4.5-fold. The stretching ratio of the secondary stretchingis preferably from 1-fold to 4-fold, more preferably from 1.2 to2.5-fold, and further preferably from 1.2 to 2.3-fold.

Further, there is also no particular restriction on the stretchingspeed, and usually the principal stretching speed and the secondarystretching speed are regulated according to the ratio. Morespecifically, if the principal stretching ratio is set at 2-fold thesecondary stretching ratio, the principal stretching speed is often setat 2-fold the secondary stretching. The stretching speed may be set at ausually applied speed without particular restriction, and is oftenregulated to a speed, which does not cause breakage of a film during thestretching.

If a polymeric piezoelectric material is stretched solely by a tensileforce as in the cases of a uniaxial stretching process or a biaxialstretching process, the stretching temperature of a polymericpiezoelectric material is preferably in a range of 10° C. to 20° C.higher than the glass transition temperature of a polymericpiezoelectric material.

When a pre-crystallized sheet is stretched, the sheet may be preheatedimmediately before stretching so that the sheet can be easily stretched.

Since the preheating is performed generally for the purpose of softeningthe sheet before stretching in order to facilitate the stretching, thesame is normally performed avoiding conditions that promotecrystallization of a sheet before stretching and make the sheet stiff.

Meanwhile, as described above, in the first embodimentpre-crystallization is performed before stretching, and therefore thepreheating may be performed combined with the pre-crystallization.Specifically, by conducting the preheating at a higher temperature thana temperature normally used, or for longer time conforming to theheating temperature or the heat treatment time at the aforementionedpre-crystallized step, preheating and pre-crystallization can becombined.

[Annealing Treatment Step]

From a viewpoint of improvement of the piezoelectric constant, apolymeric piezoelectric material after a stretching treatment shouldpreferably be subjected to a certain heat treatment (hereinafter alsoreferred to as an “annealing treatment”). The temperature of anannealing treatment is preferably about from 80° C. to 160° C. and morepreferably from 100° C. to 155° C.

There is no particular restriction on a method for applying a hightemperature in an annealing treatment, examples thereof include a directheating method using a hot air heater or an infrared heater, and amethod, for dipping a polymeric piezoelectric material in a heatedliquid such as silicone oil. In this case, if a polymeric piezoelectricmaterial is deformed by linear expansion, it becomes practicallydifficult to obtain a flat film, and therefore high temperature isapplied preferably under application of a certain tensile stress (e.g.0.01 MPa to 100 MPa) on a polymeric piezoelectric material to preventthe polymeric piezoelectric material from sagging.

The high temperature application time at an annealing treatment ispreferably from 1 sec to 60 min, more preferably from 1 sec to 300 sec,and further preferable is heating for from 1 sec to 60 sec. If annealingcontinues beyond 60 min, the degree of orientation may sometimesdecrease due to growth of spherocrystals from molecular chains in anamorphous part at a temperature above the glass transition temperatureof a polymeric piezoelectric material, and as the result thepiezoelectricity may sometimes deteriorate.

A polymeric piezoelectric material treated for annealing as describedabove is preferably quenched after the annealing treatment.

In connection with an annealing treatment, “quench” means that apolymeric piezoelectric material treated for annealing is dipped, forexample, in ice water immediately after the annealing treatment andchilled at least to the glass transition point Tg or lower, and betweenthe annealing treatment and the dipping in ice water, etc. there is noother treatment.

Examples of a quenching method include a dipping method, by which apolymeric piezoelectric material treated for annealing is dipped in acooling medium, such as water, ice water, ethanol, ethanol or methanolcontaining dry ice, and liquid nitrogen; a cooling method, by which aliquid with the low vapor pressure is sprayed for chilling byevaporation latent heat thereof.

For chilling continuously a polymeric piezoelectric material, quenchingby contacting a polymeric piezoelectric material with a metal rollregulated at a temperature below the glass transition temperature Tg ofthe polymeric piezoelectric material is possible. The number of quenchesmay be once or two times or more; or annealing and quenching can berepeated alternately. Further, if a polymeric piezoelectric materialhaving received the stretching treatment is subjected to the annealing,the polymeric piezoelectric material may be shrunk after the annealingcompared to before the annealing.

Second Embodiment

The second embodiment of a process for producing a polymericpiezoelectric material according to the present invention includes astep of stretching a sheet containing the optically active polymer(preferably a sheet in an amorphous state) mainly in a uniaxialdirection and a step of an annealing treatment, in the order mentioned.

In the second embodiment the step of stretching mainly in a uniaxialdirection is a step of conducting at least the principal stretching(according to need secondary stretching is further carried out).

The respective conditions for the step of stretching mainly in auniaxial direction and the step of an annealing treatment in the secondembodiment are regulated appropriately so that the crystallinity of apolymeric piezoelectric material to be produced becomes from 20% to 80%and that the product of the standardized molecular orientation MORc andthe crystallinity becomes from 25 to 250.

Moreover, preferable conditions for the step of stretching mainly in auniaxial direction and the step of an annealing treatment in the secondembodiment are respectively same as the conditions for the second stepand the annealing treatment step in the first embodiment.

In the second embodiment it is not necessary to provide a first step(pre-crystallization step) in the first embodiment.

<Use of Polymeric Piezoelectric Material>

Since the polymeric piezoelectric material according to the presentinvention is a piezoelectric material having a high piezoelectricconstant d₁₄ and superior transparency and longitudinal tear strength,as described above, the same can be used in various fields including aloudspeaker, a headphone, a touch panel, a remote controller, amicrophone, a hydrophone, an ultrasonic transducer, an ultrasonicapplied measurement instrument, a piezoelectric vibrator, a mechanicalfilter, a piezoelectric transformer, a delay unit, a sensor, anacceleration sensor, an impact sensor, a vibration sensor, apressure-sensitive sensor, a tactile sensor, an electric field sensor, asound pressure sensor, a display, a fan, a pump, a variable-focusmirror, a sound insulation material, a soundproof material, a keyboard,acoustic equipment, information processing equipment, measurementequipment, and a medical appliance.

In this case, a polymeric piezoelectric material according to thepresent invention is preferably used as a piezoelectric element havingat least two planes provided with electrodes. It is enough if theelectrodes are provided on at least two planes of the polymericpiezoelectric material. There is no particular restriction on theelectrode, and examples thereof to be used include ITO, ZnO, IZO(registered trade marks), and an electroconductive polymer.

Further, a polymeric piezoelectric material according to the presentinvention and an electrode may be piled up one another and used as alaminated piezoelectric element. For example, units of an electrode anda polymeric piezoelectric material are piled up recurrently and finallya principal plane of a polymeric piezoelectric material not covered byan electrode is covered by an electrode. Specifically, that with tworecurrent units is a laminated piezoelectric element having anelectrode, a polymeric piezoelectric material, an electrode, a polymericpiezoelectric material, and an electrode in the mentioned order. Withrespect to a polymeric piezoelectric material to be used for a laminatedpiezoelectric element, at least one layer of polymeric piezoelectricmaterial is required to be made of a polymeric piezoelectric materialaccording to the present invention, and other layers may not be made ofa polymeric piezoelectric material according to the present invention.

In the case that plural polymeric piezoelectric materials according tothe present invention are included in a laminated piezoelectric element,if an optically active polymer contained in a polymeric piezoelectricmaterial according to the present invention in a layer has L-formoptical activity, an optically active polymer contained in a polymericpiezoelectric material in another layer may be either of L-form andD-form. The location of polymeric piezoelectric materials may beadjusted appropriately according to an end use of a piezoelectricelement.

For example, if the first layer of a polymeric piezoelectric materialcontaining as a main component an L-form optically active polymer islaminated intercalating an electrode with the second polymericpiezoelectric material containing as a main component an L-formoptically active polymer, the uniaxial stretching direction (principalstretching direction) of the first polymeric piezoelectric materialshould preferably cross, especially orthogonally cross, the uniaxialstretching direction (principal stretching direction) of the secondpolymeric piezoelectric material so that the displacement directions ofthe first polymeric piezoelectric material and the second polymericpiezoelectric material can be aligned, and that the piezoelectricity ofa laminated piezoelectric element as a whole can be favorably enhanced.

On the other hand, if the first layer of a polymeric piezoelectricmaterial containing as a main component an L-form optically activepolymer is laminated intercalating an electrode with the secondpolymeric piezoelectric material containing as a main component anD-form optically active polymer, the uniaxial stretching direction(principal stretching direction) of the first polymeric piezoelectricmaterial should preferably be arranged nearly parallel to the uniaxialstretching direction (principal stretching direction) of the secondpolymeric piezoelectric material so that the displacement directions ofthe first polymeric piezoelectric material and the second polymericpiezoelectric material can be aligned, and that the piezoelectricity ofa laminated piezoelectric element as a whole can be favorably enhanced.

Especially, if a principal plane of a polymeric piezoelectric materialis provided with an electrode, it is preferable to provide a transparentelectrode. In this regard, a transparent electrode means specificallythat its internal haze is 40% or less (total luminous transmittance is60% or more).

The piezoelectric element using a polymeric piezoelectric materialaccording to the present invention may be applied to the aforementionedvarious piezoelectric devices including a loudspeaker and a touch panel.A piezoelectric element provided with a transparent electrode isfavorable for applications, such as a loudspeaker, a touch panel, and anactuator.

EXAMPLES

The embodiment of the present invention will be described below in moredetails by way of Examples, provided that the current embodiment is notlimited to the following Examples to the extent not to depart from thespirit of the embodiment.

Example 1

A polylactic acid-type resin (Registered trade mark LACEA, H-400;weight-average molecular weight Mw: 200,000; made by Mitsui Chemicals,Inc.) was charged into an extruder hopper, heated to 220° C. to 230° C.,extruded through a T-die, and contacted with a cast roll at 50° C. for0.3 min to form a 230 μm-thick pre-crystallized sheet(pre-crystallization step). The crystallinity of the pre-crystallizedsheet was measured to find 4%.

The obtained pre-crystallized sheet was stretched biaxiallysimultaneously with heating at 80° C. to 3.0-fold in the TD direction bya tenter method (principal stretching) and to 2.0-fold in the MDdirection by a roll-to-roll method (secondary stretching) to obtain afilm (stretching step).

The film after the stretching step was contacted with rolls heated to145° C. by a roll-to-roll method to perform an annealing treatment, andquenched to produce a polymeric piezoelectric material (annealingtreatment step). In this regard, the quenching was performed bycontacting the film, after the annealing treatment, with air at 20° C.to 30° C., and further contacting the same with metallic rolls of a filmwinding machine to rapidly lower the film temperature to close to roomtemperature.

Examples 2 to 8, and Comparative Examples 1 to 2

Polymeric piezoelectric materials of Example 2 to Example 8 andComparative Examples 1 to 2 were produced identically, except that thepre-crystallization conditions and the stretching conditions in theproduction of a polymeric piezoelectric material in Example 1 werechanged to those described in Table 1.

In Examples 5 to 8, the principal stretching direction was set in the MDdirection, and the secondary stretching direction in the TD direction.

TABLE 1 Pre- crystallization Pre- Anneal- conditions crystal- ing Heat-lized condi- Physical properties of resin Heat- ing sheet Stretchingconditions tions Optical ing temper- Crystal- Temper- Temper- Chiral-Mw/ purity time ature linity Ratio Ratio ature Width ature ChillingResin ity Mw Mn (% ee) (min) (° C.) (%) Process (TD) (MD) (° C.) (mm) (°C.) conditions Example 1 LA L 200000 2.87 98.5 0.3 50 4 Simultaneous3.0  2.0 80 300 145 Quenched biaxial stretching Example 2 LA L 2000002.87 98.5 0.3 50 4 Simultaneous 3.5  2.0 80 350 145 Quenched biaxialstretching Example 3 LA L 200000 2.87 98.5 0.3 50 4 Simultaneous 4.0 2.0 80 400 145 Quenched biaxial stretching Example 4 LA L 200000 2.8798.5 0.3 50 4 Simultaneous 4.0  2.0 90 400 145 Quenched biaxialstretching Example 5 LA L 200000 2.87 98.5 0.3 50 4 Simultaneous 1.2 3.0 80 120 110 Quenched biaxial stretching Example 6 LA L 200000 2.8798.5 0.3 50 4 Simultaneous 1.39 3.0 80 139 110 Quenched biaxialstretching Example 7 LA L 200000 2.87 98.5 0.3 50 4 Simultaneous 1.394.0 80 139 110 Quenched biaxial stretching Example 8 LA L 200000 2.8798.5 0.3 50 4 Simultaneous 1.39 3.5 80 139 110 Quenched biaxialstretching Comparative LA L 200000 2.87 98.5 0.3 50 4 Uniaxial 1.0  3.370 100 130 Quenched Example 1 stretching Comparative LA L 200000 2.8798.5  0.24 40 3 Uniaxial 1.0  5.6 73 100 100 Quenched Example 2stretching

Measurement of Amounts of L-form and D-form of Resin (Optically ActivePolymer)

Into a 50 mL Erlenmeyer flask 1.0 g of a weighed-out sample (polymericpiezoelectric material) was charged, to which 2.5 mL of IPA (isopropylalcohol) and 5 mL of a 5.0 mol/L sodium hydroxide solution were added.The Erlenmeyer flask containing the sample solution was then placed in awater bath at the temperature of 40° C., and stirred until polylacticacid was completely hydrolyzed for about 5 hours.

After the sample solution was cooled down to room temperature, 20 mL ofa 1.0 mol/L hydrochloric acid solution was added for neutralization, andthe Erlenmeyer flask was stoppered tightly and stirred well. The samplesolution (1.0 mL) was dispensed into a 25 mL measuring flask and dilutedto 25 mL with a mobile phase to prepare a HPLC sample solution

1. Into an HPLC apparatus 5 μL of the HPLC sample solution 1 wasinjected, and D/L-form peak areas of polylactic acid were determinedunder the following HPLC conditions. The amounts of L-form and D-formwere calculated therefrom.

HPLC Measurement Conditions

Column: Optical resolution column, SUMICHIRAL OA5000 (by Sumika ChemicalAnalysis Service, Ltd.)Measuring apparatus: Liquid chromatography (by Jasco Corporation)Column temperature: 25° C.Mobile phase: 1.0 mm-copper (II) sulfate buffer solution/IPA=98/2 (V/V)

Copper (II) sulfate/IPA/water=156.4 mg/20 mL/980 mL

Mobile phase flow rate: 1.0 mL/minDetector: Ultraviolet detector (UV 254 nm)

<Molecular Weight Distribution>

The molecular weight distribution (Mw/Mn) of a resin (optically activepolymer) contained in each polymeric piezoelectric material of Examplesand Comparative Examples was measured using a gel permeationchromatograph (GPC) by the following GPC measuring method.

GPC Measuring Method

Measuring apparatus: GPC-100 (by Waters)

Column: SHODEX LF-804 (by Showa Denko K.K.)

Preparation of sample: Each polymeric piezoelectric material of Examplesand Comparative Examples was dissolved in a solvent (chloroform) at 40°C. to prepare a sample solution with the concentration of 1 mg/mL.Measuring conditions: 0.1 mL of a sample solution was introduced intothe column at a temperature of 40° C. and a flow rate of 1 mL/min byusing chloroform as a solvent, and the concentration of the sample thatwas contained in the sample solution and separated by the column wasmeasured by a differential refractometer. With respect to the molecularweight of a resin, a universal calibration curve was prepared usingpolystyrene standard samples, and the weight-average molecular weight(Mw) for each resin was calculated therefrom. The measurement resultsfor resins used in Examples and Comparative Example are shown inTable 1. In Table 1 “LA stands for LACEA H-400.

<Measurement of Physical Properties and Evaluation>

With respect to each polymeric piezoelectric material of Example 1 toExample 8, and Comparative Examples 1 to 2 obtained as above, the glasstransition temperature Tg, melting point Tm, crystallinity, specificheat capacity Cp, thickness, internal haze, piezoelectric constant,MORc, and the rate of dimensional change were measured, and thelongitudinal tearing property was evaluated.

The evaluation results are shown in Table 2.

The measurements were carried out specifically as follows.

[Glass Transition Temperature Tg, Melting Point Tm, and Crystallinity]

Each 10 mg of respective polymeric piezoelectric materials of Examplesand Comparative Examples was weighed accurately and measured by adifferential scanning calorimeter (DSC-1, by Perkin Elmer Inc.) at atemperature increase rate of 10° C./min to obtain a melting endothermiccurve. From the obtained melting endothermic curve the melting point Tm,glass transition temperature Tg, specific heat capacity Cp andcrystallinity were obtained.

[Specific Heat Capacity Cp]

The amount of heat required to elevate the temperature by 1° C. per 1 gwas measured when the respective polymeric piezoelectric materials ofExamples and Comparative Examples were measured by the differentialscanning calorimeter. The measurement conditions were similar to theconditions for Tg and Tm.

[Rate of Dimensional Change]

Each polymeric piezoelectric material of Examples and ComparativeExamples was cut to a length of 50 mm in the MD direction and to 50 mmin the TD direction, to cut out a piece of 50 mm×50 mm rectangular film.The film was hanged in an oven set at 85° C. and subjected to anannealing treatment for 30 min (the annealing treatment for evaluationof the rate of dimensional change is hereinafter referred to as“annealing B”). During that procedure the length of the film rectangleside in the MD direction before and after the annealing B was measuredby calipers, and the rate of dimensional change (%) was calculatedaccording to the following expression. From the absolute value of therate of dimensional change the dimensional stability was evaluated. Ifthe rate of dimensional change is lower, it means the dimensionalstability is the higher.

Rate of dimensional change (%)=100×[(side length in the MD directionbefore annealing B)−(side length in the MD direction after annealingB)]/(side length in the MD direction before annealing B)

[Internal Haze]

“Internal haze” means herein the internal haze of a polymericpiezoelectric material according to the present invention, and measuredby a common measuring method.

Specifically, the internal haze (hereinafter also referred to as“internal haze (H1)”) of each polymeric piezoelectric material ofExamples and Comparative Examples was measured by measuring the lighttransmittance in the thickness direction. More precisely, the haze (H2)was measured by placing in advance only a silicone oil (Shin-EtsuSilicone (trade mark), grade: KF96-100CS; by Shin-Etsu Chemical Co.,Ltd.) between 2 glass plates; then the haze (H3) was measured by placinga film (polymeric piezoelectric material), whose surfaces were wetteduniformly with the silicone oil, between 2 glass plates; and finally theinternal haze (H1) of each polymeric piezoelectric material of Examplesand Comparative Examples was obtained by calculating the differencebetween the above two according to the following formula:

Internal haze (H1)=haze (H3)−haze (H2)

The haze (H2) and haze (H3) in the above formula were determined bymeasuring the light transmittance in the thickness direction using thefollowing apparatus under the following measuring conditions.

Measuring apparatus: HAZE METER TC-HIIIDPK (by Tokyo Denshoku Co., Ltd.)Sample size: Width 30 mm×length 30 mm, (see Table 2 for the thickness)Measuring conditions: According to JIS-K7105Measuring temperature: Room temperature (25° C.)

[Piezoelectric Constant d₁₄ (by Displacement Method)]

A specimen (polymeric piezoelectric material) of 40 mm×40 mm with the Agconductive layers on both planes formed was cut to a length of 32 mm inthe direction of 45° with respect to the stretching direction (MDdirection) of a polymeric piezoelectric material, and to 5 mm in thedirection perpendicular to the above 45° direction, to cut out a pieceof rectangular film in a size of 32 mm×S mm. This was used as a samplefor measuring the piezoelectric constant. A sine-wave alternatingvoltage of 10 Hz and 300 Vpp was applied to the prepared sample, and adifference distance between the maximum value and the minimum value ofthe displacement of the film was measured by a laserspectral-interference displacement meter SI-1000 (by KeyenceCorporation). A value obtained by dividing the measured displacement(mp-p) by the reference length of the film, which was 30 mm, was definedas strain, and a value obtained by dividing the strain by an electricfield intensity applied to the film ((applied voltage (V))/(filmthickness)) and multiplying by 2, was defined as piezoelectric constantd₁₄ (pm/V).

[Standardized Molecular Orientation MORc]

Standardized molecular orientation MORc was measured for each ofpolymeric piezoelectric materials of Examples and Comparative Examplesby a microwave molecular orientation meter MOA-6000 by Oji ScientificInstruments. The reference thickness tc was set at 50 μm.

[Evaluation of Longitudinal Tearing Property]

The longitudinal tearing property was evaluated with respect to each ofpolymeric piezoelectric materials of Examples and Comparative Examplesby measuring the tear strength in the MD direction and the tear strengthin the TD direction respectively according to the “Right angled tearmethod” stipulated in JIS K 7128-3 “Plastics—Tear strength of films andsheets”.

With respect to the evaluation of the longitudinal tearing property,when both the tear strength in the MD direction and the tear strength inthe TD direction are high, it means that deterioration of thelongitudinal tear strength is suppressed. In other words, if at leastone of the tear strength in the MD direction and the tear strength inthe TD direction is low, it means that the longitudinal tear strength isdeteriorated.

In the measurement of the tear strength, the crosshead speed of atensile testing machine was set at 200 mm/min and the tear strength wascalculated according to the following formula:

T=F/d

wherein T stands for the tear strength (N/mm), F for the maximum tearload, and d for the thickness (mm) of a specimen.

TABLE 2 Evaluation result Rate of of longitudinal Piezo- Length dimen-tear property Crystal- Thick- Internal electric MORc × after sional MDTD Tg Cp Tm linity ness MORc haze constant Crystal- annealing changeDirection Direction (° C.) (J/g° C.) (° C.) (%) (μm) @50 μm (%) (pm/V)linity (mm) (%) (N/mm) (N/mm) Example 1 74.2 0.09 171.0 38.4 32.4 2.950.1 3.5 113 50 0.00 88.2 136.5 Example 2 74.1 0.29 172.0 37.1 37.6 4.390.3 5.8 163 49.7 0.60 91.2 150.6 Example 3 74.4 0.26 172.0 37.8 31.64.05 0 5.6 153 49.4 1.20 88.8 125.8 Example 4 74.8 0.22 172.2 39.2 36.54.04 0.2 6.4 158 49.3 1.40 84.9 144.4 Example 5 N.D. N.D. 169.3 36.346.9 4.89 0.38 5.1 177 N.D. N.D. 109.5 191.2 Example 6 N.D. N.D. 168.639.1 45.8 4.27 0.48 4.9 167 N.D. N.D. 91.9 187.1 Example 7 N.D. N.D.169.8 39.3 43.2 4.71 0.2 5.7 185 N.D. N.D. 94.0 210.5 Example 8 N.D.N.D. 168.9 40.8 43.2 4.42 0.04 4.9 180 N.D. N.D. 141.7 194.0 Comparative70.4 0.31 169.3 43.9 60.0 6.10 0.3 7.0 268 50 0.00 52.2 410.9 Example 1Comparative 77.9 0.24 167.6 47.9 43.0 5.32 17.1 5.1 255 N.D. N.D. 48.2N.D. Example 2

As seen in Table 2, in Examples 1 to 8, a deterioration of thelongitudinal tear strength (here tear strength in the MD direction) wassuppressed compared to Comparative Example 1.

Also in Examples 1 to 8, the transparency was good (namely, the internalhaze was low) and the piezoelectric constants were high (1 pm/V orhigher).

On the other hand, in Comparative Example 2, although the piezoelectricconstant was almost same as in Examples 1 to 8, the longitudinal tearstrength (here tear strength in the MD direction) deteriorated comparedto Examples 1 to 8.

Meanwhile, “N.D.” in Table 2 means that a measurement was omitted andthere is no data.

The entire disclosure of Japanese Patent Applications No. 2011-272708 isincorporated herein by reference.

All publications, patent applications, and technical standards describedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

1. A polymeric piezoelectric material comprising a helical chiralpolymer having a weight-average molecular weight of from 50,000 to1,000,000 and having optical activity, wherein a crystallinity of thematerial measured by a DSC method is from 20% to 80%, and a product of astandardized molecular orientation MORc measured by a microwavetransmission type molecular orientation meter based on a referencethickness of 50 μm and the crystallinity is from 25 to
 250. 2. Thepolymeric piezoelectric material according to claim 1, wherein thecrystallinity is 40.8% or less.
 3. The polymeric piezoelectric materialaccording to claim 1, wherein an internal haze with respect to visiblelight is 40% or less.
 4. The polymeric piezoelectric material accordingto claim 1, wherein the standardized molecular orientation MORc is from1.0 to 15.0.
 5. The polymeric piezoelectric material according to claim1, wherein a piezoelectric constant d₁₄ measured by a displacementmethod at 25° C. is 1 pm/V or higher.
 6. The polymeric piezoelectricmaterial according to claim 1, wherein the helical chiral polymer is apolylactic acid-type polymer having a main chain comprising a repeatingunit represented by the following formula (1):


7. The polymeric piezoelectric material according to claim 1, wherein anoptical purity of the helical chiral polymer is 95.00% ee or higher. 8.The polymeric piezoelectric material according to claim 1, wherein acontent of the helical chiral polymer is 80 mass % or more.
 9. Thepolymeric piezoelectric material according to claim 1, wherein aninternal haze with respect to visible light is 1.0% or less.
 10. Aprocess for producing the polymeric piezoelectric material according toclaim 1, comprising a first step of heating a sheet in an amorphousstate containing the helical chiral polymer to obtain a pre-crystallizedsheet, and a second step of stretching the pre-crystallized sheetsimultaneously in biaxial directions.
 11. The process for producing thepolymeric piezoelectric material according to claim 10, wherein, in thefirst step for obtaining the pre-crystallized sheet, the sheet in anamorphous state is heated at a temperature T satisfying the followingformula until the crystallinity becomes between 1% and 70%:Tg−40° C.≦T≦Tg+40° C. wherein Tg represents a glass transitiontemperature of the helical chiral polymer.
 12. The process for producingthe polymeric piezoelectric material according to claim 10, wherein, inthe first step for obtaining the pre-crystallized sheet, the sheet in anamorphous state containing polylactic acid as the helical chiral polymeris heated at from 20° C. to 170° C. for from 5 sec to 60 min.
 13. Theprocess for producing the polymeric piezoelectric material according toclaim 10, wherein an annealing treatment is conducted after the secondstep.